Method For Operating A System For Producing Bioethanol

ABSTRACT

The invention relates to a method for operating a system for producing bioethanol, wherein organic waste products of the production process, particularly DGS and DDGS, are combusted and the useful heat is fed back into the system itself. The combustion process takes place in a fluidized bed oven. All areas in which the combustion process takes place have sufficient heat removed that the melting point of the ash of the waste product, particularly 70° C., is not exceeded at any point. In this manner, a fine-grained ash forms and largely mixes into the fluidized bed and is easily disposed of. The useful heat is obtained partially from the flue gas arising from the combustion, and partially from the heat extracted from the combustion process for maintaining the maximum temperature.

The invention relates to a method for operating a system for producing bioethanol, in which organic waste products from the production process, in particular DGS and DDGS, are combusted and the useful heat obtained in this way is returned to the system itself.

It is known, in systems for producing bioethanol, to combust the organic waste products which arise in the form of solubles or also in the form of dried granular products, in particular those which are known as DGS and DDGS, and to re-use the useful heat obtained in this way in the system itself. This can be seen for example from the online dictionary Wikipedia, as at October 2008, under the head word “Bioethanol”. This literature reference does not say anything about the type of combustion process or the temperatures which occur during this.

The types of disposal for DGS and DDGS which are currently most common are those of cattle feed, fertiliser, substrate in biogas plants and combustion in biomass heating power plants. With these types of disposal, the organic waste products are transported from the site of the bioethanol system to the disposal site.

The object of the present invention is to provide a method of the type mentioned at the outset such that overall the plant for producing bioethanol can be operated more efficiently and the organic waste products can be disposed of at lower cost.

This object is achieved according to the invention in that

-   a) the waste products are combusted in a fluidised bed combustor,     and sufficient heat is removed from all the chambers in which the     combustion process takes place for the melting point of the ash of     the waste products, in particular 700° C., not to be exceeded at any     location; -   b) the useful heat is obtained partly from the flue gases that arise     during combustion and partly from the heat removed from the     combustion process in order to ensure observance of the maximum     temperature.

The invention is based on the fundamental idea, to be found in the above-mentioned Wikipedia literature reference, of not disposing of the organic waste products from the bioethanol production process at a different location but combusting it on site and using the useful heat obtained in this way in the system itself. There are many ways of using heat in a bioethanol production system, whether to generate steam or to heat system parts or materials directly.

The invention goes beyond this known fundamental idea and proposes carrying out the combustion process in a fluidised bed combustor. If the combustion process is performed therein such that the melting point of the ash of the waste products is not exceeded at any location, a fine-grained solid ash is obtained which mixes into the fluidised bed and can be disposed of from there without difficulty. If no heat were removed from the combustion process as proposed by the invention, the temperature would increase to a value at which the ash produced melted. However, it is very much more difficult to dispose of liquid ash than fine-grained solid ash such as that produced in the method according to the invention.

Nor is the cooling of the combustion chamber, proposed according to the invention, to a temperature below the melting point of the ash of the waste products accompanied by any significant drop in thermal efficiency, since the heat that is removed is used, like the heat in the flue gases themselves.

Because of the relatively low temperatures prevailing in the fluidised bed combustor in the method according to the invention, it is possible to dispense with a refractory lining for the housing of the fluidised bed combustor. This not only has considerable cost advantages, but because of the smaller masses, the fluidised bed combustor can be started up and cooled down very much more quickly than when there is a lining.

As already indicated above, an advantageous embodiment of the method according to the invention consists in using the useful heat at least partly to generate steam.

To remove the heat from the combustion process, at least one heat transfer means through which a heat transfer medium flows can be used. The geometry and site of installation of the heat transfer means are selected in dependence on the respective local conditions such that the objective of ensuring observation of a maximum temperature in the combustion process is achieved.

The heat transfer medium may be water. This variant is particularly advantageous where direct steam generation is sought.

As an alternative, the heat transfer medium may be a thermal oil. This thermal oil is first used to remove heat from the combustion process; this heat may then be used in any way desired.

For energy reasons, it may be advantageous if the air used to generate the fluidised bed is pre-heated by the flue gases exiting the fluidised bed combustor.

Waste products which would ignite if introduced above the fluidised bed should always be introduced directly into the fluidised bed so that they are distributed as evenly as possible and are completely combusted.

In the case of waste products which do not ignite on being introduced above the fluidised bed, a distinction needs to be made between cases: if their nature would cause them to rise in the gas flow above the fluidised bed, they should also be introduced directly into the fluidised bed. If, however, their nature causes them to sink in the gas flow above the fluidised bed, they should be introduced above the fluidised bed. In making this selection of the location of introducing the waste products, in each case the best possible mixing and combustion of the waste products have to be considered.

An exemplary embodiment of the invention will be explained in more detail below with reference to the drawing; the single FIGURE shows diagrammatically a system for performing the method according to the invention.

The system, which is designated overall by the reference numeral 1, includes as a main component a fluidised bed combustor 2, whereof the fundamental structure is known. Its housing 3 is composed of three coaxial sections 3 a, 3 b, 3 c which are all rotationally symmetrical. The bottom section 3 a is cylindrical; this is adjoined above by a conically widening section 3 b, above which, finally, there is mounted a section 3 c which is once again cylindrical. The housing 3 is made of a steel which is resistant to extremely high temperature and has a wall thickness of approximately 10 to 15 mm and is not provided with a refractory lining as is generally the case with known fluidised bed combustors of this type.

The bottom region 3 a of the housing 3 is divided into two chambers by a horizontal jet plate 4. The lower chamber 4 a serves as an air chamber. A fan 5 draws air from the outside atmosphere by suction, for reasons to be explained below optionally directly through a line 25 or by way of a heat transfer means 13, and guides this air into the air chamber 4 a. A burner 6, which is connected to the air chamber 4 a, is fed with natural gas, as the combustion gas, through a schematically indicated line 7, and with combustion air 8.

A fluidised bed 9 is provided in the upper chamber 4 b, projecting as far as the section 3 b of the housing 3 and comprising a granular, inert and heat-resistant material, in particular sand. The waste product from the bioethanol production which is to be combusted (which may be DGS or DDGS) can be introduced into the interior of the housing 3 above the upper level of the fluidised bed 9, through a schematically illustrated line 10. As mentioned above, this location for introduction is particularly suitable for those waste products which are relatively moist and heavy and are not readily ignited.

The inner chamber 11 of the housing 3 which lies above the fluidised bed 9 serves as a calming chamber. The hot flue gases can be guided away from this through a line 12 and fed via the heat transfer means 13 to a steam generator 14. The steam generator 14 may be of any known construction, which does not need to be described in detail. Water enters the steam generator 14 through a line 26; the end product of the process that is sought in the present case, namely hot steam, exits through a line 16 and the cooled flue gas exits through a line 16 and can then be fed to a flue.

Heat transfer means 17, 18 are mounted both in the fluidised bed 9 and in the free inner chamber 11 of the housing 3 above the fluidised bed 9. The heat transfer means 17, 18 may take any desired form provided they fulfil the following: during combustion of the waste products they must be capable of cooling the entire combustion chamber of the housing 3, that is to say both the space occupied by the fluidised bed 9 and the free chamber 11 above it, to a temperature below the melting point of the ash of these waste products. It is imperative to keep reliably below this temperature in a manner as constant as possible in the entire inner chamber of the combustor 3. In the case of the combustion of DGS or DDGS which is under consideration here, this means that the temperature must not exceed approximately 650 to 700° C. at any location. The manner in which the heat transfer means 17, 18 are constructed for this purpose can be established by simple tests of the respective geometry of the fluidised bed combustor 2 and the waste products which are respectively to be combusted.

The heat transfer means 17, 18 are in a heat transfer circuit in which a thermal oil is kept circulating by means of a pump 19. A circulation line 20 leads to this, starting from the compression side of the pump 19 and leading first of all via the heat transfer means 18 and then to the lower heat transfer means 17. From there, the thermal oil is taken on through the line 20 to a heat transfer means 21 which is itself accommodated inside the steam generator 14, where heat is removed from the thermal oil and emitted to aid the process of steam generation. Then the cooled thermal oil returns to the pump 19.

The system 1 described above is operated as follows:

When the system 1 is started up, first of all air is fed to the air chamber 4 a with the aid of the fan 5, this air being drawn by suction from the ambient air, through the line 22 and either through the line 25 or the heat transfer means 13. Because the heat transfer means 13 is still cold at this point, the air drawn in by suction is first of all at ambient temperature in both cases.

The air blown into the air chamber 4 flows through the jet plate 4 and fluidises the bed of sand which lies above it, with the result that the actual fluidised bed 9 is formed.

With the aid of the burner 6, to which natural gas and combustion air are fed through the lines 7 and 8 respectively, the air blown into the combustion chamber of the combustor 3 through the jet plate 4, and hence also the fluidised bed 9, are heated to a temperature at which the waste product from the bioethanol production, which is now introduced through the line 10, begins to burn. If this waste product carries sufficient energy, then operation of the burner 6 can be reduced or switched off completely after this starting phase.

With the aid of the heat transfer means 17, 18, through which the pump 19 pumps thermal oil, a temperature which is below the melting point of the ash of the waste product is maintained inside the fluidised bed 9 and the free chamber 11 above it, in the manner already described above.

This means that over time the amount of solid, fine-grained and pourable ash in the fluidised bed 9 increases. Consequently, during operation the height of the fluidised bed 9 grows. By measuring the pressure drop in the fluidised bed 9 with the aid of suitable sensors, the point at which the fluidised bed 9 reaches a defined height that is to not to be exceeded is detected. At that point, material comprising a mixture of ash and sand is removed from the fluidised bed 9 through the line 27 by a suitable removal mechanism (not illustrated). This mixture can be separated again if desired, in a manner not described here in more detail, such that the ash can be disposed of and the sand if required returned to the fluidised bed 9.

The flue gases which leave the fluidised bed combustor 2 through the line 12 may also take with them relatively small quantities of ash particles. These may if required be removed from the hot flue gases by a cyclone which is located in the line 12 but is not illustrated in the drawing. These flue gases pass through the heat transfer means 13 and now pre-heat the air which is drawn in by suction through the line 22 by the fan 5 and blown into the air chamber 4 a of the fluidised bed combustor 2. As the flue gases continue on their path they reach the steam generator 14, where they are cooled, generating steam which exits through the line 15, such that they can be emitted to the outside atmosphere through the line 16, as relatively cool flue gases.

The heat coming from the combustion process and removed from the fluidised bed combustor 2 by the heat transfer means 17, 18 is brought to the heat transfer means 21 inside the steam generator 14 via the circulation line 20. There, it in turn contributes to steam generation. 

1. A method for operating a system for producing bioethanol, in which organic waste products from the production process, in particular DGS and DDGS, are combusted and useful heat obtained in this way is returned to the system itself, comprising the steps of: combusting the waste products in a fluidised bed combustor, and removing sufficient heat is removed from all chambers in which the combustion process takes place so that a melting point of an ash of the waste products is not exceeded at any location; and obtaining useful heat partly from flue gases that arise during combustion and partly from the heat removed from the combustion process in order to ensure observance of the maximum temperature.
 2. The method of claim 1, wherein the useful heat is used at least partly to generate steam.
 3. The method of claim 1, wherein to remove the heat from the combustion process, at least one heat transfer means through which a heat transfer medium flows is used.
 4. The method of claim 3, wherein the heat transfer medium is water.
 5. The method of claim 3, wherein the heat transfer medium is a thermal oil.
 6. The method of claim 1, wherein the air used to generate a fluidised bed is pre-heated by the flue gases exiting the fluidised bed combustor.
 7. The method of claim 1, wherein waste products which would ignite if introduced above a fluidised bed are introduced directly into the fluidised bed.
 8. The method of claim 1, wherein waste products which would not ignite on being introduced above a fluidised bed and whereof the nature would cause them to rise in a gas flow above the fluidised bed are introduced directly into the fluidised bed.
 9. The method of claim 1, wherein waste products which do not ignite on being introduced above a fluidised bed and whereof the nature causes them to sink in a gas flow above the fluidised bed are introduced above the fluidised bed. 